APPLICANT'S DESCRIPTION: Class switch DNA recombinations are important for isotype switching in B cells and also appear to be involved in chromosomal translocations of some oncogenes. However, little is known about the switching mechanism. Tandemly-repeated sequences located upstream of all H-chain antibody constant regions genes have generally been thought to be important in the targeting and/or the mechanism of class switch recombination. We have used gene targeting to delete the tandem repeats from the switch (S) region associated with the Cu constant region in mice. Surprisingly, these mutant mice are still able to undergo isotype switching, although the efficiency of the process is modestly reduced. Our findings indicate that the Su. tandem repeats are not required for the switching process. The current proposal seeks to extend our preliminary results to analyze several aspects of class switch DNA recombination. It has been reported that DNA mismatch repair (MMR) enzymes also affect the efficiency and joining site selection of class switching. We wish to investigate the mutual relationships between the Su tandem repeats and MMR enzymes in the switching process by producing double-mutant animals that lack both the Su repeats and specific individual MMR proteins. Second, we want to determine whether RNA:DNA complexes can form in the ASu JH-Cu intron that lacks the tandem repeat element to explore the previously suggested importance of these complexes in the switching process. Finally, to investigate the relative importance of the Su tandem repeats, in comparison to the tandem repeats found associated with downstream CH genes, we will produce, by gene targeting, mutant mice that lack either Sa or both Su and Sa. Analysis of switching in these mutant mice should indicate whether downstream S regions are much more important in controlling the recombination process as has been suggested by some studies of artificial switch substrates.